1. Field of the Invention
The invention relates to an economical process for making molten ferrous alloy of the stainless steel type, and more particularly to such a process utilizing a charge consisting primarily of chrome and iron ores which have been partially reduced and converted to a suitable physical form for charging into a submerged-arc furnace.
2. Brief Description of the Prior Art
Heretofore, prior art workers have produced stainless steel through the use of an electric arc melting furnace, an oxygen converter or a combination of the two. These procedures have been further modified by the use of a vacuum in a degasser, ladle or other steel making vessel to assist in the removal of undesirable gases and to aid in the refining process.
The charge into the furnace or converter generally consisted of stainless steel scrap which had to have been segregated or classified prior to its use, carbon steel scrap, a small quantity of iron ore, fluxing agents and ferro-alloys. One of the primary ferro-alloys used was a ferro-chromium which could be of either the low or the high carbon variety. Ferro-chromium, particularly the low carbon variety, is an expensive source of chromium.
Through the years, improvements have been made in techniques, apparatus and atmosphere control. However, such improvements frequently resulted in new problems which had to be overcome. For the most part, the high cost of the charging ingredients and the labor costs for classifying scrap added considerably to the high refining and production costs of stainless steel. As a consequence, the use of stainless steel has been limited, despite the many advantages it has over carbon steel or low cost materials.
U.S. Pat. No. 3,728,101 made a significant contribution to the art because it teaches a process for the production of stainless steel utilizing an initial charge consisting primarily of iron ore and chrome ore. This charge is introduced into a first vessel and an unrefined liquid ferrous alloy is produced by means of a submerged-arc. The hot, liquid ferrous alloy is transferred to a second vessel and subjected to two gaseous treatments, the first comprising blowing pure gaseous oxygen against the liquid alloy and the second comprising the introduction of pure gaseous oxygen and an inert gas or nitrogen into the liquid alloy below the surface thereof. Finishing additions and temperature adjustments are made in the second vessel to achieve the desired final chemistry and tap temperature. When the steel is to have a nickel content, nickel is added during one or both of the gas purifying steps. When a high nickel content is desired, nickel may also be added to the submerged-arc furnace (first vessel).
A primary drawback of the process of U.S. Pat. 3,728,101 lies in the fact that a single first vessel in the form of a submerged-arc furnace cannot provide a sufficient output to economically charge the second vessel, generally in the form of a A/O vessel. Since chrome ores are considerably more difficult to reduce than iron ores, the normal size submerged-arc furnace, as known today, can be relied upon to produce only about five tons or less per hour of the unrefined alloy. The typical A/O vessel requires a charge in the neighborhood of about 20 tons per hour. Therefore, to practice the process of U.S. Pat. 3,728,101 properly, the output of some four or five submerged-arc furnaces are required to charge the typical A/O vessel (i.e. the second vessel).
The present invention constitutes an improvement in the process of the above mentioned U.S. Pat. 3,728,101 in that it renders the process of that patent economically more feasible. In accordance with the present invention, the iron and chrome ores making up the primary portion of the charge to the first vessel (submerged-arc furnace) are pre-reduced and rendered in an appropriate physical form for charging into the submerged-arc furnace. As will be discussed hereinafter, this pre-reduction step can be accomplished in a number of well known ways.
Together with the pre-reduction step, the present invention contemplates rendering the chrome and iron ores in a physical form appropriate for charging into a submerged-arc furnace. By "appropriate form" is meant that the ores should be intimately mixed and should not contain significant amounts of powder or fines. To this end, briquetting, nodulizing, pelletizing and sintering (all of which are well known in the art) may be employed. For purposes of the present specification and the claims hereafter, the generic term "agglomeration" will be used and is intended to include all of the methods listed above. In some instances, sized ores may be employed in the submerged-arc furnace. While, as will be appreciated by one skilled in the art, sizing is not truly an agglomeration procedure, for purposes of the present specification and claims the term "agglomeration" is to be construed broadly enough to include sizing.
As will be developed further hereinafter, the agglomeration of the ores may be accomplished before, during or after the pre-reduction step. Some pre-reduction methods require the use of previously agglomerated ores. Other pre-reduction methods utilize a fluidized bed, and agglomeration is accomplished after the pre-reduction step. Yet other pre-reduction methods result simultaneously in the agglomeration of the ores as a part of the pre-reduction procedure.
The pre-reduction of the chrome and iron ores should be as complete as possible, consistent with optimal time and economic considerations. As indicated above, it is known that iron ores will reduce before chrome ores thermodynamically. The present invention contemplates a pre-reduction of iron ores of up to about 95% or more and a pre-reduction of the chrome ores of from about 20% to about 60% or more.
The practice of the present invention provides advantages in addition to rendering the process of U.S. Pat. 3,728,101 more economic. First of all, the present process enables the use of low grade chrome ores (having a chrome to iron ratio of 3.5 or less) such as Transvaal type ores, hitherto deemed undesirable. The practice of the present invention places such typically friable low grade chrome ores in an improved form for the submerged-arc furnace. Such low grade chrome ores are easier to mine, available in greater quantities than high grade chrome ores and are far less expensive. The present invention enables the use of these low grade ores for up to 100% of the chrome requirements. As a consequence, the present process enables more economical use of more raw materials and less scrap.
The operation of a submerged-arc electric furnace is expensive. Through the practice of the present invention, depending upon the type of pre-reduction step used, the ores may be charged into the submerged-arc furnace already hot, thus reducing the operating cost of the furnace. As a result of the present invention, the productivity of the submerged-arc furnace (which operates on a continuous basis) is markedly increased. The process of U.S. Pat. 3,728,101 may be practiced utilizing only one or two submerged-arc furnaces for each A/O vessel.